Explainer · updated 2026-11-09
Urea (BUN), explained in plain English
Key facts
- Typical adult reference interval: 8-20 mg/dL (ABIM)
- What urea nitrogen is: the nitrogen waste left when protein is broken down, cleared by the kidneys (Gounden 2024)
- What pushes it up besides kidney disease: being short of fluid (Gounden 2024)
- What pulls it down: very little protein in the diet, starvation, and severe long-standing liver disease (Gounden 2024)
What it is
Urea nitrogen is the nitrogen waste left when protein is broken down, and the kidneys clear it. The test measures it in the blood. It moves with how much protein you eat and how much fluid you carry, so it is read next to creatinine rather than on its own.
What the number is telling you
The figure is read beside creatinine and the estimated filtration rate, and beside your fluid intake and diet. Being short of fluid concentrates it and lifts the reading without the kidneys changing.
What high or low can mean
High: It rises in both sudden and long-standing kidney disease. Being short of fluid pushes it up as well.
Low: It falls when very little protein is being eaten, in starvation, and in severe long-standing liver disease.
What a typical result looks like
Typical adult reference intervals from ABIM run from 8 to 20 mg/dL for urea nitrogen. The range on your own report is the one that counts.
Questions worth asking your doctor
- How does this read beside my creatinine?
- Could I have been short of fluid when this was taken?
- How much protein is in my diet?
- How does this compare with my previous results?
If a result worries you, ask your doctor.
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